To work out steps per unit you meed to know the line count of the encoder, the pitch of the screws and any gearing that you have between motor and screw. If you let me know that I will show you how to work out steps per unit, oh also say whether its metric or imperial units you will be using as default units.
Velocity will depend on the motor and also possibly the voltage your drive is supplied with, for example servos will have a velocity constant, in other words X amount of volts per 1000RPM. The motors will also have a max continuous RPM rating. If you give details about the motors and the supply voltage to the drives I can give you a ballpark figure.
Hood